Computer help forum: Please help me fix my computer

Hey guys, I wasn't sure where to post this so I thought I'd try here. I'm having a big problem with my new computer and I do not know very much about how to fix computers

Here are the specs:
i5-3570K CPU @ 3.40GHz
8GB DDR3
1TB Hitachi HDD
AMD Radeon HD 7850
64-bit Windows 8

Here's the problem: When I first got my computer all my games ran fantastically well. Now it has been about 2 weeks and my games have started to stutter every once and awhile (I first noticed it playing Company of Heroes). However, I was recently playing StarCraft II and my game kept on freezing for about 3 - 10 seconds and then fast forwarding to catch up. I tried lowering the graphics but this had no effect on the stutters, in fact it made them worse. It was also strange that my background programs were not affected (I was talking on Skype at the time and there was no interruption). The game was completely unplayable so I hit ctrl+alt+delete and noticed that my disk usage was at 100%. My computer continued to run extremely slowly and I keep having the stutter problem whenever I play games. I don't know what to do. I've tried reading a bunch of forums but I don't even know what the issue is and I haven't found any solutions.

Is it a Windows 8 problem? Is it an AMD issue? Is something wrong with my Hard Drive? I have no idea. What do you think is wrong? And how can I fix it?